1. Field of the Invention
Embodiments disclosed herein relate generally to a downhole mill assembly. More particularly, the embodiments disclosed herein relate to a downhole mill assembly and a method of milling using diamond enhanced elements.
2. Background Art
When an existing cased oil well becomes unproductive, the well may be sidetracked in order to develop multiple production zones or redirect exploration away from an unproductive zone. Generally, sidetracking involves the creation of a window in the well casing by milling the steel casing in an area either near the bottom or within a serviceable portion of the well. The milling operation is then followed by the directional drilling of rock formation through the newly formed casing window. Sidetracking enables the development of a new borehole directionally oriented toward productive hydrocarbon sites without moving the rig, platform superstructure, or other above ground hole boring equipment, and also takes advantage of a common portion of the existing casing and cementing in the original borehole.
Thus, sidetracking is often preferred because drilling, casing, and cementing the borehole are avoided. As mentioned above, this drilling procedure is generally accomplished by either milling out an entire section of casing followed by drilling through the side of the now exposed borehole, or by milling through the side of the casing with a mill that is guided by a wedge or “whipstock” component.
The casing window is generally created with a combination of mills mounted on a shaft or mandrel at the bottom end of a drill string and wedging between the casing and a whipstock, which is generally set in the hole in combination with the first milling run. The peripheral surface of mills is generally covered with abrasive or cutting inserts made of hard material such as sintered tungsten carbide compounds brazed on a steel shaft. The hardness of the whipstock is generally designed so that minimum wear will be generated by the rotation of mills peripheral surface onto the whipstock face while the assembly is pushed and rotated against the casing wall under deflecting action of the whipstock. The milling action generally results from unbalanced pressures between the mill(s) and the whipstock on one hand and the mill(s) and the casing wall on the other hand.
U.S. Pat. No. 4,266,621, which is herein incorporated by reference, describes a milling tool for elongating a laterally directed window in a well casing. The disclosed system requires three trips into the well, beginning with the creation of an initial window in the borehole casing, the extension of the initial window within a particular cutting tool, and the elongation and further extension of the window by employing an assembly with multiple mills. While the window mill is aggressive in opening a window in the casing, the number of trips, typically three, to accomplish the task is expensive and time consuming.
By integrating a whipstock into the milling operation and directionally orienting the milling operation to a more confined area of well casing, the number of trips required to effectively mill a window in a well casing has been decreased. A whipstock having an acutely angled ramp is first anchored inside a well and properly oriented to direct a drill string in the appropriate direction. A second trip is required to actually begin the milling operations. Newer methods integrate the whipstock with the milling assembly to provide a combination whipstock and staged sidetrack mill, allowing for casing windows to be milled in one trip. The milling assembly is connected at its leading tool to the top portion of the whipstock by a bolt, which upon application of sufficient pressure, may be sheared off to free the milling assembly. The cutting tool employed to mill through the metal casing of the borehole has conventionally incorporated cutters that include at least one material layer, such as preformed or crushed tungsten carbide, designed to mill pipe casing. Several such one-trip milling systems include those described in U.S. Pat. Nos. 5,771,972, 6,102,123, 6,648,068, which are herein incorporated by reference in their entirety.
Conventional milling systems are, however, unable to mill windows in chrome casings, casings which are steadily increasing in number of wells due to the number of wells in severe drilling environments, such as severely corrosive environments, deep wells, cold environments, and sea bottoms, that are more commonly drilled due to exhaustion of easily drillable wells. The presence of and exposure to corrosive fluid, necessitates the use of corrosion resistant alloys (CRA), frequently duplex chrome, in the downhole components including casings. Other typically corrosion and/or erosion resistant CRA-type materials include: (1) stainless steel including conventional austenitic, martensitic, precipitation hardened, duplex, and ferritic stainless steels; (2) precipitation hardened and solid solution nickel-based alloys and nickel copper alloys; and (3) cobalt-based, titanium, and zirconium alloys. Although desired corrosion resistance can be obtained, for example, using a 25% duplex chrome, the material proves to be difficult in handling, specifically, in cutting and machining. The material tends to be abrasive to cutting tools, as well as leading to work hardening, smearing, galling, and welding. The difficulties associated with milling through a chrome casing leaves many mature wells neighbored by significant quantities of oil otherwise unreachable without the cost of either pulling the chrome casings and recompleting the existing well or forming a new well. The ability to sidetrack a well would not only allow for a multilateral well, but would also allow for sidetracking of a stuck drill string.
Furthermore, as the mill cutting structure meets the casing wall, the cutting structure typically encounters severe vibrations that frequently lead to cracks in the cutters. Such cracks may lead to failure of the cutters, reducing the life of the mill and likely preventing the mill from being used to drill through the sidetracked, secondary borehole (through the earth formation) after the window is milled.
However, further improvements in milling systems would allow for increased longevity of mills in an operational environment that frequently leads to failure of mills by cracking of cutters.